Method of manufacturing a wind turbine blade

ABSTRACT

The present invention relates to a method of manufacturing a wind turbine blade, comprising arranging one or more layers of fibre material and a preform in a mould ( 66 ), injecting the one or more layers of fibre material and the preform ( 76 ) with a curable resin, and curing the resin. The preform ( 76 ) is impregnated with a curing promoter such that the concentration of curing promoter varies spatially within the preform.

FIELD OF THE INVENTION

The present invention relates to a method of manufacturing a windturbine blade, to a preform of an elongate reinforcing element for awind turbine blade and to a wind turbine blade obtainable by the methodof the present invention.

BACKGROUND OF THE INVENTION

The rotor blades of modern wind turbines capture kinetic wind energy byusing sophisticated blade design created to maximise efficiency. Thereis an increasing demand for large wind blades which may exceed 80 metresin length and 4 metres in width. The blades are typically made from afibre-reinforced polymer material and comprise a pressure side shellhalf and a suction side shell half. The cross-sectional profile of atypical blade includes an airfoil for creating an air flow leading to apressure difference between both sides. The resulting lift forcegenerates torque for producing electricity.

The shell halves of wind turbine blades are usually manufactured usingblade moulds. First, a blade gel coat or primer is applied to the mould.Subsequently, fibre reinforcement and/or fabrics are placed into themould followed by resin infusion. The resulting shell halves areassembled by being glued or bolted together substantially along a chordplane of the blade.

Vacuum infusion or VARTM (vacuum assisted resin transfer moulding) isone method, which is typically employed for manufacturing compositestructures, such as wind turbine blades comprising a fibre-reinforcedmatrix material. During the manufacturing process, liquid polymer, alsocalled resin, is filled into a mould cavity, in which fibre material hasbeen arranged, and where a vacuum is generated in the mould cavityhereby drawing in the polymer. The polymer can be thermoset plastic orthermoplastics. Typically, uniformly distributed fibres are layered in afirst rigid mould part, the fibres being rovings, i.e. bundles of fibresarranged in mats, felt mats made of individual fibres or unidirectionalor woven mats, i.e. multi-directional mats made of fibre rovings, etc. Asecond mould part, which is often made of a resilient vacuum bag, issubsequently placed on top of the fibre material and sealed against thefirst mould part in order to generate a mould cavity. By generating avacuum, typically 80 to 95% of the total vacuum, in the mould cavitybetween the first mould part and the vacuum bag, the liquid polymer canbe drawn in and fill the mould cavity with the fibre material containedherein.

Resin transfer moulding (RTM) is a manufacturing method, which issimilar to VARTM. In RTM the liquid resin is not drawn into the mouldcavity due to a vacuum generated in the mould cavity. Instead the liquidresin is forced into the mould cavity via an overpressure at the inletside.

In the above-described manufacturing process, preforms may be used. Apreform is a shaped arrangement of fibres, such as one or multiplelayers thereof, which has been bound and/or consolidated for later useas part of the fibre lay-up in the blade mould. The rationale for usingpreforms for blade manufacturing is to reduce cycle time in the blademould. Also, using preforms may reduce the number of required repairsdue to the pre-consolidated structure of the preforms.

Preforms may be used for producing blade parts such as a reinforcingelements, such as main laminates or spar cap, for forming a load-bearingstructure of a wind turbine blade. However, when producing large blades,the main laminate gets proportionally larger, also in terms of itsoverall volume. Since such parts usually have a comparatively lowthickness towards their edges this may lead to problems during curing.In particular, this may lead to undesired differences in curingtemperatures within the main laminate. Thinner parts of the element maynot receive significant exotherm heat, thus not heating up as much asthicker, central parts of the reinforcing element. This may lead tomanufacturing defects within the part as the degree and timing ofshrinkage during the curing process may vary spatially, i.e. the degreeand timing of shrinkage in the thicker parts may be different from thedegree and timing of shrinkage in the edge portions.

It is thus a first object of the present invention to provide a methodof manufacturing a wind turbine blade or parts thereof resulting in lessmanufacturing defects, in particular of reinforcing blade elements.

It is a further object of the present invention to provide a method ofmanufacturing a wind turbine blade which is cost-effective and flexible.

It is another object of the present invention to provide a method ofresin-infusing and curing of a wind turbine blade part which results inless thermal stress and related deformations in the finished part.

SUMMARY OF THE INVENTION

The present inventors have found that one or more of said objects may beachieved by a method of manufacturing a wind turbine blade, the bladehaving a profiled contour including a pressure side and a suction side,and a leading edge and a trailing edge with a chord having a chordlength extending therebetween, the wind turbine blade extending in aspanwise direction between a root end and a tip end, said methodcomprising:

-   providing a mould,-   arranging one or more layers of fibre material in the mould for    providing a skin element,-   arranging a preform on at least part of the one or more layers of    fibre material for providing a reinforcing element,-   injecting the one or more layers of fibre material and the preform    with a curable resin, and-   curing the resin,

wherein the preform is impregnated with a curing promoter such that theconcentration of curing promoter varies spatially within the preform.

The present inventors have found that the use of such preforms enablethe production of comparatively large reinforcing elements, such as mainlaminates, with few to no manufacturing defects. It was also found thatthis approach results in a reduced cycle time for curing. The curingpromoter may advantageously accelerate the curing process locallytowards the edges of the preform. The present inventors have found thisto yield a surprisingly even and balanced curing profile leading to lessmanufacturing defects caused by an uneven/irregular curing process. Insome embodiments, the preform may be impregnated with the curingpromoter along both its lateral edges within longitudinally extendingstrips of 5-200 mm width, such as 10-100 mm width, adjacent to either ofits lateral edges, preferably such that the remainder of the preform isnot impregnated with the curing promoter.

The mould may comprise a moulding surface corresponding substantially tothe outer surface of a wind turbine blade shell half. Typically, thestep of arranging one or more layers of fibre material in the mould forproviding a skin element includes laying several layers of fibrematerial successively onto the moulding surface of the mould. The fibrematerial may comprise glass fibres, carbon fibres or a combinationthereof. According to a preferred embodiment of the method, a glassfibre material is placed into the mould, such as multiple layers ofglass fibre material, for providing a skin element. The fibre materialmay optionally be brought into contact with a binding agent before orduring the fibre lay-up. The fibre lay-up process may involve aligning aplurality of fibres, or fibre layers, substantially unidirectionally. Inone embodiment, the fibre material may include fibre rovings, such asglass fibre rovings. The lay-up process may include placing multiplesingle roving bundles into the mould, the roving bundles beingpreferably aligned unidirectionally.

The preform is then arranged on at least part of the one or more layersof fibre material for providing a reinforcing element. In someembodiments, the skin element may comprise a recess for receiving thepreform of a reinforcing element. Typically, the preform will compriseat least one fibre material, such as a glass fibre material. In apreferred embodiment, the preform comprises at least one fibre layerand/or fibre fabric, such as two or more fibre layers and/or fibrefabrics. Preferably, the preform comprises strands of parallel fibres,such as glass fibres and/or carbon fibres. In some embodiments, two ormore of the preforms according to the present invention are arranged inthe mould.

Next, the one or more layers of fibre material and the preform areinjected with a curable resin. The resin used according to the presentinvention is preferably a thermosetting resin. Examples of suitablethermosetting resins include ester-based resins, such as unsaturatedpolyester resins, vinyl ester resins and urethane (meth)acrylates.Typically, the resin infusion step comprises vacuum assisted resintransfer moulding. The resin may be an epoxy, a polyester, a vinyl esteror another suitable thermoplastic or duroplastic material, preferably avinyl ester. In other embodiments, the resin may be a thermosettingresin, such as epoxy, vinyl ester or polyester, or a thermoplasticresin, such as nylon, PVC, ABS, polypropylene or polyethylene.

The resin is then cured, preferably with no external heating.Advantageously, redox systems are used for resin curing. Such redoxsystems may comprise an oxidizing agent, such as a peroxide and asoluble transition metal ion as curing accelerator. The curingaccelerator preferably increases the activity of the oxidizing agent atlower temperatures, such as at ambient temperatures, thus enhancing thecuring rate.

The preform of the reinforcing element is impregnated with a curingpromoter such that the concentration of curing promoter varies spatiallywithin the preform. Thus, in embodiments in which the preform comprisesa fibre material, the fibre material of the preform is impregnated withthe curing promoter such that the concentration of curing promotervaries spatially within the fibre material of the preform.

Preferably, the preform is an elongate preform. In a preferredembodiment, the preform is impregnated with the curing promoter prior tolayup in the mould. For instance, an elongate preform may be wound up ona roll or spindle prior to layup, wherein the two opposing outer edgesof the wound-up preform could be contacted with the curing promoter. Inother embodiments, the reform is impregnated with a curing promotor oflayup in the mould.

The method may also comprise the steps of curing the resin to form anupwind blade half and/or a downwind blade half, and joining an upwindblade half and a downwind blade half to form a wind turbine blade.

In a preferred embodiment, the preform has a cross section with acentral portion and two opposing outer edges, wherein the thickness ofthe preform decreases from the central portion towards each of the twoouter edges, and wherein the preform is impregnated with the curingpromoter such that the concentration of curing promoter decreases fromone or both outer edges towards the central portion of the preform. Thepreform may have a length, i.e. a longitudinal extension, a width and athickness or height. Typically, its width and thickness are less thanits length. For example, the width could be 0.5-1.5 m, the thicknesscould be 1-100 mm and the length could be 10 m or more. The length orlongitudinal extension of the preform and of the resulting reinforcingelement, such as a main laminate, will usually coincide with thespanwise direction of the wind turbine blade. A cross section of thepreform will usually extend between a left outer edge and a right outeredge, the distance between said left and right outer edges defining thewidth of the preform at that cross section. A central portion of suchcross section may correspond to a midpoint of the distance between saidleft and right outer edges, or to a portion or interval including suchmidpoint. For example, the central portion may extend at least 50 mm,such as at least 100 mm, or at least 200 mm, from the midpoint towardseach of the opposing outer edges in the width direction of the preform.The decrease in concentration of curing promoter from one or both outeredges towards the central portion of the preform may be an abruptdecrease from a given concentration to zero. In other, embodiments itmay be a continuous or smooth decrease.

In a preferred embodiment, the preform has two edge regions, each edgeregion extending laterally within a distance of 100 mm or less from therespective outer edge towards the central portion of the preform,wherein the preform is impregnated with the curing promoter within oneor both edge regions. Said edge regions preferably extend over theentire length of the preform. It is preferred that the preform isimpregnated with the curing promoter within both edge regions,advantageously over the entire length of the preform. In someembodiments, the concentration of the curing promoter is constant withinboth edge regions. Preferably, the concentration of the curing promoteris substantially constant within the entire volume of the respectiveedge region or strip. In other embodiments, the concentration of thecuring promoter, such as a curing accelerator, is zero in the centralportion of the preform.

The two edge regions will typically be opposing edge regions, such as aleft edge region and a right edge region, as seen in a cross section ofthe preform. Each edge region may extend laterally within a distance of10 mm or less, 20 mm or less, 30 mm or less, 40 mm or less, 50 mm orless, 60 mm or less, 70 mm or less, 80 mm or less, or 90 mm or less,from the respective outer edge towards the central portion or midpointof the preform, as seen in a cross section of the preform, preferablyover the entire length of the preform. As used herein, extendinglaterally means an extension in the horizontal (width) direction H ofthe preform, as illustrated e.g. in FIG. 7 .

Usually, the central portion of the preform includes the midpoint of thepreform on a line connecting a left and a right outer edge as seen in across section of the preform. In some embodiments, the central portionextends along the entire width of the preform except for the two edgeregions.

According to another embodiment, the preform is not impregnated with thecuring promoter outside of said edge regions. In other embodiments, theconcentration of the curing promoter is lower outside said edge regionsthan within said edge regions. In some embodiments, the concentration ofthe curing promoter is non-zero outside said edge regions.

In a preferred embodiment, the preform is impregnated with the curingpromoter prior to the step of arranging the preform on at least part ofthe one or more layers of fibre material. Part of the fibre material ofthe preform may be contacted with the curing promoter to achieveimpregnation, the impregnated material may then be dried and stored forfurther use.

In some embodiments, the curing promoter is a curing acceleratorcomprising a metal. The curing accelerator of the present invention maycomprise one or more salts of metals such as lithium, calcium, copper,vanadium, zirconium, titanium, zinc, iron, sodium, potassium, magnesium,manganese, barium and cobalt, optionally in combination with one or morecompounds of alkyl organic acids, halides, nitrates to form acoordination compound. A preferred curing accelerator according to thepresent invention may comprise a transition metal salt or complex. In apreferred embodiment, the transition metal is cobalt. In someembodiments, the curing accelerator may be premixed to form a metal saltcomplex prior to contacting the preform with the curing accelerator. Inother embodiments, individual components of the curing accelerator maybe contacted with the preform separately to form the metal complex insitu. In a preferred embodiment, the curing promoter is a curingaccelerator comprising a transition metal such as cobalt, manganese,iron or copper. In other embodiments, the curing accelerator comprisesone or more organic cobalt salts and/or one or more amines, such astertiary amines.

In some embodiments, the curing accelerator may be added to the preformbefore adding a curing initiator such as a peroxide. In otherembodiments, the preform may be impregnated with the curing acceleratoronly, prior to infusing the resin in the fibre parts. In someembodiments, the preform may be contacted with a solution containing thecuring accelerator to impregnate the preform with the same.

In other embodiments, the curing promoter is a curing initiator such asa peroxide, preferably an organic peroxide. In some embodiments, thecuring promotor is a combination of a curing initiator such as aperoxide, preferably an organic peroxide, and a curing accelerator, suchas one or more organic cobalt salts and/or one or more tertiary amines.

In a preferred embodiment, the resin comprises a polyester, such as anunsaturated polyester. An unsaturated polyester resin may be cured byfree radicals which are formed when organic peroxides decompose. Thedecomposition initiates a reaction by which unsaturated polyestermolecules polymerize with styrene forming a three-dimensional structure.Organic peroxides may decompose into free radicals by exposure to heator in combination with one or more curing accelerators.

According to another embodiment, the curing of the resin is performedwithout external heating. Thus, the curing may be a cold curing,activated by a curing accelerator, such as an amine and/or acobalt-containing compound. Useful curing systems include aremethylethylketone, cyclohexanone or acetylacetone peroxides incombination with a cobalt-containing compound such as organic cobaltsalts, and dibenzoyl peroxide in combination with one or more tertiaryamines.

In a preferred embodiment, the reinforcing element is a load-carryingmain laminate or spar cap of the wind turbine blade for supporting oneor more shear webs. The main laminate is typically formed as a fibreinsertion which comprises a plurality of fibre reinforcement layers,e.g. between 20 and 50 layers. However, the preforms could also be usedfor other parts and regions of a wind turbine blade, such as reinforcedparts of the leading edge and/or the trailing edge of the blade.

In another aspect, the present invention relates to a preform of anelongate reinforcing element for a wind turbine blade, the preformcomprising a fibre material, wherein the preform is impregnated with acuring promoter such that the concentration of curing promoter variesspatially within the preform. In a preferred embodiment, the preformcomprises at least one fibre layer or fibre fabric. Preferably, thepreform comprises strands of parallel fibres, such as glass fibresand/or carbon fibres.

According to another embodiment, the preform has a cross section with acentral portion and two opposing outer edges, wherein the thickness ofthe preform decreases from the central portion towards each of the twoouter edges, and wherein the preform is impregnated with the curingpromoter such that the concentration of curing promoter decreases fromone or both outer edges towards the central portion of the preform.

In a preferred embodiment, the preform has two edge regions, each edgeregion extending laterally within a distance of 100 mm or less from therespective outer edge towards the central portion of the preform,wherein the preform is impregnated with the curing promoter within oneor both edge regions. As described above, each edge region may extendlaterally within a distance of 10 mm or less, 20 mm or less, 30 mm orless, 40 mm or less, 50 mm or less, 60 mm or less, 70 mm or less, 80 mmor less, or 90 mm or less, from the respective outer edge towards thecentral portion or midpoint of the preform, as seen in a cross sectionof the preform.

According to another embodiment, the preform is not impregnated with thecuring promoter outside of said edge regions. In some embodiments, thepreform is not impregnated with the curing accelerator, as describedherein, outside of said edge regions.

In a preferred embodiment, the curing promoter is present in the edgeregion in a concentration of 0.01 to 10 parts, such as 0.01 to 1 parts,or 0.1 to 1 parts, by weight of curing promoter relative to the weightof the fibre material. In other embodiments, the curing promoter,preferably the curing accelerator, is present outside of the edgeregions in a concentration of 0 to 0.1, such as 0 to 0.01 parts byweight of the fibre material. Preferably, the concentration of curingpromoter, preferably curing accelerator, is at least five times, atleast ten times or at least 25-times higher within the edge regions ascompared to outside the edge regions. In some embodiments, the curingpromoter is present within parts of the preform, such as the edgeregions, in an amount of 0.01-15 wt%, such as 0.1-15 wt% or 0.1-5 wt%,relative to the weight of the fibre material.

According to another embodiment, the curing promoter is a curingaccelerator comprising a transition metal such as cobalt, manganese,iron or copper. In a preferred embodiment, the curing promoter is acuring initiator such as a peroxide, preferably an organic peroxide.Preferably, the curing initiator is methyl ethyl ketone peroxide.

The preform of the present invention may further comprise a bindingagent which is added to the fibre material for an improved handling.Such binding agent is preferably present in an amount of 0.1-15 wt%relative to the weight of the fibre material. The binding agent may alsobe present in an amount of 5-40, preferably 10-20, gram per square meterof glass surface. In some embodiments, the binding agent is athermoplastic binding agent. The binding agent may comprise a polyester,preferably a bisphenolic polyester.

In one embodiment of the present invention, the preform has a length ofbetween 5 and 100 meters. In some embodiments, the preform has a lengthof at least 5, 10, 20 or 50 meters.

In another aspect, the present invention relates to a wind turbine bladeobtainable by the above-described method. Such wind turbine blade wasfound to be more resilient and to exhibit less manufacturing defectsresulting from thermal stress created in the curing process of thereinforcing element.

It will be understood that any of the above-described features may becombined in any embodiment of the inventive method or preform. Inparticular, features and embodiments described with regard to thepreform may also apply to the method of manufacturing, and vice versa.

As used herein, the term “longitudinal” means the axis runningsubstantially parallel to the maximum linear dimension of the preform,the reinforcing element or the blade.

As used herein, the term “substantially” usually means what isspecified, but may deviate from the specified amount by 15% or less, 10%or less or 5% or less.

As used herein, the term “wt%” means weight percent. The term “relativeto the weight of the fibre material” means a percentage that iscalculated by dividing the weight of an agent, such as a curingpromoter, by the weight of the fibre material. As an example, a value of1 wt% relative to the weight of the fibre material corresponds to 10 gof curing promoter per kilogram of fibre material.

As used herein, the term “concentration” refers to a measure of anamount or weight of a substance, such as a curing promoter, containedper weight of dry material of the preform, such as the weight of fibrematerial, within a given region of the preform.

As used herein, the term “elongate” or “elongate preform” refer to apreform having two dimensions that are much less than a third dimension,such as at least three, five, ten or twenty times less than a thirddimension. Typically, the third dimension will be the length(longitudinal extension) of the preform, as opposed to the two lesserdimensions, width and height (thickness). The length (longitudinalextension) of the preform will typically be in the spanwise direction ofthe wind turbine blade.

DETAILED DESCRIPTION OF THE INVENTION

The invention is explained in detail below with reference to embodimentsshown in the drawings, in which FIG. 1 shows a wind turbine,

FIG. 2 shows a schematic view of a wind turbine blade,

FIG. 3 shows a schematic view of an airfoil profile through section I-Iof FIG. 4 ,

FIG. 4 shows a schematic view of the wind turbine blade, seen from aboveand from the side,

FIG. 5 is a schematic cross-sectional view of a mould for moulding ablade part according to the present invention,

FIG. 6 is a perspective view of a preform according to the presentinvention,

and FIG. 7 shows a cross sectional view of the preform taken along theline A-A′ in FIG. 6 .

DETAILED DESCRIPTION

FIG. 1 illustrates a conventional modern upwind wind turbine accordingto the so-called “Danish concept” with a tower 4, a nacelle 6 and arotor with a substantially horizontal rotor shaft. The rotor includes ahub 8 and three blades 10 extending radially from the hub 8, each havinga blade root 16 nearest the hub and a blade tip 14 furthest from the hub8.

FIG. 2 shows a schematic view of a first embodiment of a wind turbineblade 10 according to the invention. The wind turbine blade 10 has theshape of a conventional wind turbine blade and comprises a root region30 closest to the hub, a profiled or an airfoil region 34 furthest awayfrom the hub and a transition region 32 between the root region 30 andthe airfoil region 34. The blade 10 comprises a leading edge 18 facingthe direction of rotation of the blade 10, when the blade is mounted onthe hub, and a trailing edge 20 facing the opposite direction of theleading edge 18.

The airfoil region 34 (also called the profiled region) has an ideal oralmost ideal blade shape with respect to generating lift, whereas theroot region 30 due to structural considerations has a substantiallycircular or elliptical cross-section, which for instance makes it easierand safer to mount the blade 10 to the hub. The diameter (or the chord)of the root region 30 may be constant along the entire root area 30. Thetransition region 32 has a transitional profile gradually changing fromthe circular or elliptical shape of the root region 30 to the airfoilprofile of the airfoil region 34. The chord length of the transitionregion 32 typically increases with increasing distance r from the hub.The airfoil region 34 has an airfoil profile with a chord extendingbetween the leading edge 18 and the trailing edge 20 of the blade 10.The width of the chord decreases with increasing distance rfrom the hub.

A shoulder 40 of the blade 10 is defined as the position, where theblade 10 has its largest chord length. The shoulder 40 is typicallyprovided at the boundary between the transition region 32 and theairfoil region 34.

It should be noted that the chords of different sections of the bladenormally do not lie in a common plane, since the blade may be twistedand/or curved (i.e. pre-bent), thus providing the chord plane with acorrespondingly twisted and/or curved course, this being most often thecase in order to compensate for the local velocity of the blade beingdependent on the radius from the hub.

FIGS. 3 and 4 depict parameters which are used to explain the geometryof the wind turbine blade according to the invention. FIG. 3 shows aschematic view of an airfoil profile 50 of a typical blade of a windturbine depicted with the various parameters, which are typically usedto define the geometrical shape of an airfoil. The airfoil profile 50has a pressure side 52 and a suction side 54, which during use - i.e.during rotation of the rotor - normally face towards the windward (orupwind) side and the leeward (or downwind) side, respectively. Theairfoil 50 has a chord 60 with a chord length c extending between aleading edge 56 and a trailing edge 58 of the blade. The airfoil 50 hasa thickness t, which is defined as the distance between the pressureside 52 and the suction side 54. The thickness t of the airfoil variesalong the chord 60. The deviation from a symmetrical profile is given bya camber line 62, which is a median line through the airfoil profile 50.The median line can be found by drawing inscribed circles from theleading edge 56 to the trailing edge 58. The median line follows thecentres of these inscribed circles and the deviation or distance fromthe chord 60 is called the camber f. The asymmetry can also be definedby use of parameters called the upper camber (or suction side camber)and lower camber (or pressure side camber), which are defined as thedistances from the chord 60 and the suction side 54 and pressure side52, respectively.

Airfoil profiles are often characterised by the following parameters:the chord length c, the maximum camber f, the position d_(f) of themaximum camber f, the maximum airfoil thickness t, which is the largestdiameter of the inscribed circles along the median camber line 62, theposition d_(t) of the maximum thickness t, and a nose radius (notshown). These parameters are typically defined as ratios to the chordlength c. Thus, a local relative blade thickness t/c is given as theratio between the local maximum thickness t and the local chord lengthc. Further, the position d_(p) of the maximum pressure side camber maybe used as a design parameter, and of course also the position of themaximum suction side camber.

FIG. 4 shows other geometric parameters of the blade. The blade has atotal blade length L. As shown in FIG. 3 , the root end is located atposition r = 0, and the tip end located at r = L. The shoulder 40 of theblade is located at a position r= L_(w), and has a shoulder width W,which equals the chord length at the shoulder 40. The diameter of theroot is defined as D. The curvature of the trailing edge of the blade inthe transition region may be defined by two parameters, viz. a minimumouter curvature radius r_(o) and a minimum inner curvature radius r_(i),which are defined as the minimum curvature radius of the trailing edge,seen from the outside (or behind the trailing edge), and the minimumcurvature radius, seen from the inside (or in front of the trailingedge), respectively. Further, the blade is provided with a prebend,which is defined as Δy, which corresponds to the out of plane deflectionfrom a pitch axis 22 of the blade.

FIG. 5 is a schematic cross-sectional view through a mould 66 for use ina method of manufacturing a wind turbine blade. The mould comprises amoulding surface 68, which defines an outer surface of the finished windturbine blade, here shown as the pressure side of the blade.

A number of fibre layers, core parts and reinforcement sections arearranged on the moulding surface 68, these parts forming a skin element70 of the aerodynamic shell part or pressure side shell part 72 of thewind turbine blade (details not shown). The aerodynamic shell part 72may for instance be manufactured by first applying a waxy substance tothe moulding surface in order to be able to remove the shell part aftermoulding. Also, a gelcoat may be applied to the moulding surface. Theskin element may comprise a recess 74 for receiving a preform of areinforcing element 76, such as a spar cap or main laminate. The preformof the reinforcing element 76 extends in a longitudinal direction of theblade and forms a load carrying structure of the finished blade afterresin infusion and curing.

The preform 76 has a cross section with a central portion 78 and twoopposing outer edges 80, 82. The thickness of the preform 76 decreasesfrom the central portion 78 towards each of the two outer edges 80, 82.Preferably prior to arranging the preform 76 in the mould, it isimpregnated with a curing promoter such that the concentration of curingpromoter decreases from one or both outer edges 80, 82 towards thecentral portion 78 of the preform. The skin element 70 and the preform76 are injected with a curable resin which is then cured to form thewind turbine blade part 72.

FIG. 6 is a perspective view of a preform 76 of the present invention,wherein FIG. 7 shows a cross-sectional view of the preform 76 takenalong the line A-A′ in FIG. 6 , substantially perpendicular to thelongitudinal direction LO indicated in FIG. 6 . As indicated by theshaded areas in FIG. 6 , the preform 76 is impregnated with the curingpromoter along both its lateral edges 80, 82 within longitudinallyextending strips or edge regions 81, 83 adjacent to the lateral edges80, 82. Preferably, the remainder of the preform 76 is not impregnatedwith the curing promoter.

As best seen in FIG. 7 , the preform 76 has a thicker central portion 78and two edge regions 81, 83, each edge region extending laterally withina distance E1, E2 of, for example, 100 mm or less from the respectiveouter edge 80, 82 towards the central portion 78 of the preform 76. Thelower part of FIG. 7 illustrates a concentration profile 84 across thepreform 76 extending between both outer edges 80, 82, wherein theconcentration c_(p) of the curing promoter is plotted versus thehorizontal distance d from outer edge 80. The vertical and horizontaldimensions V, H are also indicated in FIG. 7 . As seen in the graph ofFIG. 7 , the preform 76 is impregnated with curing promoter within theedge regions 81, 83, while the preform 76 is not impregnated with thecuring promoter outside of said edge regions 81, 83. While aconcentration profile 84 is illustrated in FIG. 7 , a smoother or moretransient concentrations profile is also possible according to thepresent invention.

List of reference numerals 2 wind turbine 4 tower 6 nacelle 8 hub 10blade 14 blade tip 16 blade root 18 leading edge 20 trailing edge 22pitch axis 30 root region 32 transition region 34 airfoil region 40shoulder / position of maximum chord 50 airfoil profile 52 pressure side54 suction side 56 leading edge 58 trailing edge 60 chord 62 camber line/ median line 66 mould 68 moulding surface 70 skin element 72 shell part74 recess 76 preform of reinforcing element 78 central portion ofpreform 80 first outer edge 81 first edge region 82 second outer edge 83second edge region 84 concentration profile c chord length c_(p)concentration of curing promoter d distance d_(t) position of maximumthickness d_(f) position of maximum camber d_(p) position of maximumpressure side camber E1, E2 distances from outer edge f camber Hhorizontal direction L blade length LO longitudinal direction r localradius, radial distance from blade root t thickness V vertical directionΔy prebend

1. A method of manufacturing a wind turbine blade, the blade having aprofiled contour including a pressure side and a suction side, and aleading edge and a trailing edge with a chord having a chord lengthextending therebetween, the wind turbine blade extending in a spanwisedirection between a root end and a tip end, said method comprising:providing a mould (66); arranging one or more layers of fibre materialin the mould for providing a skin element (70); arranging a preform (76)on at least part of the one or more layers of fibre material forproviding a reinforcing element; injecting the one or more layers offibre material and the preform (76) with a curable resin; and curing theresin, wherein the preform (76) is impregnated with a curing promotersuch that the concentration of curing promoter varies spatially withinthe preform (76).
 2. The method according to claim 1, wherein thepreform (76) has a cross section with a central portion (78) and twoopposing outer edges (80, 82), wherein the thickness of the preform (76)decreases from the central portion (78) towards each of the two outeredges (80, 82), and wherein the preform (76) is impregnated with thecuring promoter such that the concentration of curing promoter decreasesfrom one or both outer edges (80, 82) towards the central portion (78)of the preform (76).
 3. The method according to claim 1, wherein thepreform (76) has two edge regions (81, 83), each edge region extendinglaterally within a distance of 100 mm or less from the respective outeredge (80, 82) towards the central portion (78) of the preform (76),wherein the preform (76) is impregnated with the curing promoter withinone or both edge regions (81, 83).
 4. The method according to claim 1,wherein the preform (76) is impregnated with the curing promoter priorto the step of arranging the preform (76) on at least part of the one ormore layers of fibre material.
 5. The method according to claim 1,wherein the preform (76) comprises at least one fibre layer or fibrefabric.
 6. The method according to claim 1, wherein the curing promoteris a curing accelerator comprising a transition metal such as cobalt,manganese, iron or copper.
 7. The method according to claim 1, whereinthe curing promoter is a curing initiator such as a peroxide, preferablyan organic peroxide.
 8. The method according to claim 1, wherein theresin comprises a polyester, such as an unsaturated polyester.
 9. Themethod according to claim 1, wherein the curing of the resin isperformed without external heating. 11-15. (canceled)
 16. The methodaccording to claim 3, wherein the preform (76) is not impregnated withthe curing promoter outside of said edge regions (81, 83).